Whenever a multi-species plasma is generated using certain materials, it can happen that the resultant plasma will contain both positive and negative ions. This result is particularly possible when the material being ionized is a chemical compound which contains a halogen element, or an element such as oxygen or sulfur. As is well known, these elements all have a relatively high electron affinity and, consequently, the neutral atoms of these elements are quite easily joined with free electrons to create negative ions. On the other hand, these same elements also have a relatively high ionization potential and, therefore, electrons are not so easily detached from the neutral atom to create a positive ion.
For applications wherein a plasma is generated from chemical compounds which include a halogen as one of the constituent elements (also consider oxygen, sulfur), it is quite possible to generate a multi-species plasma that will include both positive and negative ions. Specifically, this result can occur when the plasma is generated using an ionization potential that is below the ionization potential of the halogen (or oxygen, sulfur). If this is the case, positive ions can still be created from the other elements in the compound, but not for the halogen (oxygen, sulfur) element. Instead, the halogen (oxygen, sulfur) element will remain neutral or be subsequently converted to a negative ion.
As indicated above, neutral atoms of a halogen (oxygen, sulfur) have a relatively high electron affinity. Consequently, these elements are much more susceptible to being converted to negative ions than are elements with relatively low electron affinity. For applications wherein the objective is to separate the halogen (oxygen, sulfur) element from the positive ions of another element, this susceptibility can be of considerable concern. Specifically, although neutral atoms (uncharged particles) can be relatively easily separated from positive ions (charged particles) in a plasma, the situation is much different when the neutral atoms themselves become negative ions (charged particles). When this happens, the negative ions are not so easily separated from the positive ions. Nevertheless, there are instances when both positive and negative ions may be present in the same multi-species plasma and it would be very desirable to separate them from each other, and thereby prevent them from recombining.
In U.S. Pat. No. 6,096,220, which was filed by Ohkawa on Nov. 16, 1998 for an invention entitled "Plasma Mass Filter," and which is assigned to the same assignee as the present invention, it has been shown that charged particles in a multi-species plasma can be separated from each other according to their respective masses. In particular, it has been shown that by using specifically configured crossed electric and magnetic fields (E.times.B) in a filter chamber, positive ions of relatively small mass to charge ratios can be confined inside the chamber during their transit of the chamber. On the other hand, positive ions of relatively large mass to charge ratios would not be so confined. Instead, these larger mass ions would be collected inside the chamber before completing their transit through the chamber.
Using the same general principles previously disclosed in Ohkawa's earlier invention for separating positive ions of different mass, the present invention has recognized that by appropriately modifying the crossed electric and magnetic fields (E.times.B) in a filter chamber, negative ions and positive ions can be separated from each other. More specifically, in this case, the positive ions in a multi-species plasma can be confined inside a plasma filter chamber during their transit of the filter chamber, while the negative ions in the plasma are expelled into the wall of the filter chamber.
In light of the above it is an object of the present invention to provide a plasma filter, and a method for its use, which is capable of separating positive ions from negative ions when both types of ions are present in the same multi-species plasma. Another object of the present invention is to provide a plasma filter, and a method for its use, that can effectively prevent positive ions from recombining with negative ions when both type ions are present in the same multi-species plasma. Yet another object of the present invention is to provide a plasma filter, and a method for its use, that expands the principles of plasma mass filter technology to multi-species plasma having both positive ions and negative ions in the plasma. Still another object of the present invention is to provide a plasma filter that is relatively easy to manufacture, is simple to use, and is comparatively cost effective.